Electrical resistivity of plated metals is of foremost significance and concern in circuit fabrication, such as in the construction of printed and integrated circuits. Achievement of the lowest possible electrical resistance in the conductive current paths with accompanying circuit integrity are among the more important and continuing goals. These characteristics can be obtained only through close monitoring and control during the metal deposition processes. Successful, uniform plating capability has developed into a highly skilled art.
The resistivity of metals is influenced by many factors, including material lattice structure, impurity content and temperature. In circuit fabrication by additive plating processes, copper is one of the primary metals used because of its relatively low resistivity and favorable ductility. In additive processes, the metal is selectively deposited on a substrate by immersion in a plating bath that may be either an autocatalytic electroless bath, or an electroplating bath. The autocatalytic bath process is slow with copper, for instance, being deposited at the rate of 0.002 millimeters per hour. The bath formulation and plating process are critical to plating performance and are carefully controlled to insure and maintain bath stability, plating uniformity, and desired electrical and mechanical characteristics of the metal.
Electrical resistivity, a measure of the metal's ability to conduct-an electric current, can be separated into two components. One is the ideal temperature-dependent resistivity due to lattice vibrations, and the other is the residual resistivity due to impurities and lattice imperfections. At any given temperature, the ideal temperature-dependent resistivity is constant but the total resistance can vary among specimens of the same metal depending upon the magnitude of the residual resistance. It then becomes apparent that total resistivity can be changed only by altering the residual resistivity component.
This residual resistivity can be decreased in smelted metals by further refinement to improve the purity. Plated metals are not amenable to this processing and their residual resistivities have not been generally considered a controllable property.